It is known in the art to use a dished-shaped snap acting thermostatic element such as a bimetallic disc with a so-called movable electrical contact welded to the disc as a contact assembly. Many of these contact assemblies. are part of motor protector devices or the like in which the dished-shaped thermostatic bimetallic element provides actuation means for the device. The protector devices are located typically directly adjacent or inside the motor or other electrical equipment that the device is protecting to provide inherent protection which senses not only over current conditions but also over temperature conditions. Since the contact assemblies are current carrying, the amount of current flowing through the thermostatic element, providing self-heating, as well as the ambient temperature can result in a fault condition to cause the element to snap to an inverted dished-shaped configuration moving the movable contact away from a stationary contact of the device.
In one type of protector device a movable electrical contact is attached to one portion of a thermostatic disc element in a conventional manner, e.g., as by welding thereto, with another portion of the disc element attached to a first housing member of the protector device, as by welding thereto. The movable contact is adapted to move into and out of engagement with a stationary electrical contact mounted on a second housing member electrically separated from the first housing member with the first and second housing members electrically connected to respective terminals. Under normal operating conditions the contacts are in engagement with one another thereby closing an electrical circuit between the terminals but upon being heated to a predetermined temperature the snap acting disc element moves a first dished configuration to an opposite second dished configuration thereby moving the movable contact out of engagement with the stationary contact to open the electrical circuit between the terminals. This opening of the circuit serves to prevent the apparatus being protected from being damaged due to over temperature conditions.
In attaching the thermostatic disc element to a support it is conventional to weld a member commonly called a slug to the disc element blank at the location of the disc element to be later welded to the support. The disc element blank can then be formed into the dished configuration of the disc element to impart snap acting characteristics responsive to selected temperatures. The slug then aids in allowing the disc element to be welded to the support without significantly affecting the snap acting disc element characteristics by minimizing the heat affected zone of the thermostatic metals of the disc element. The weld slug generally has a plurality of projections extending from a surface and spaced inwardly from an outer periphery so that the actual location of the fused metal will be pin-pointed requiring less energy and so that much of the heat caused by the welding operation will be absorbed by the slug to facilitate the limitation of the heat affected zone of the metal layers of the disc element and minimize any changes of the temperature calibration values and other characteristics of the disc element such as the amount of movement of the disc element and the available force generated during such movement. One type of weld slug in wide use has a post extending from the slug which is received through an aperture provided in the disc element to precisely locate the slug relative to the disc element and so that the post can then be welded to the support. In this way the effects of the energy used in welding the post to the support on the heat affected zone of the disc member can be minimized.
This arrangement also allows the provision of clearance between the thermostatic disc element and the support to avoid transfer of any forces from the support so that it is possible to provide a fully characterized disc assembly wherein the desired characteristics of the assembly can be formed therein prior to installation in a device and then the disc assembly can be placed in any one of various devices resulting in consistent, predictable disc element characteristics from one device to another. Without this clearance between the disc assembly and the support the disc element characteristics are dependent upon the device in which the disc assembly is received as well as the installation procedure employed. This results in varying characteristics from one device to another and the need for additional efforts in sorting of devices requiring reworking and in yield.
Disc assemblies which can be characterized in terms of how the disc element will behave independently of the device in which the assemblies are ultimately mounted represent a significant advantage; however, there has been a problem associated with such assemblies.
A typical disc assembly comprises an electrical contact and a slug welded to a disc element on the high expansion side of the element. The useful life of this assembly is limited generally by the development and propagation of cracks in the thermostatic disc element. These cracks initiate in the heat affected zones of the disc element proximate to the location of attachment to the housing member. As the disc element continues to cycle on and off the cracks develop and propagate and eventually affect the temperature at which the disc element changes from one configuration to the opposite configuration taking the disc element out of the useful calibration range. When this occurs the device no longer provides the desired protection and must be replaced.
An object of the present invention is to provide an improved fully characterized snap acting thermostatic disc assembly of the type described having enhanced cycle life. Another object is the provision of a method for increasing the useful life of a snap acting thermostatic disc element adapted for cantilever mounting to a support structure. Yet another object of the invention is the provision of an electrical equipment protector having an improved thermostatic disc assembly which is reliable in operation and economical to produce.
Briefly, a thermostatic disc assembly made in accordance with the invention comprises a thermostatic snap acting disc element movable between a first concave curved configuration and a reversed second convex curved configuration in dependence upon the temperature of the disc element having a plate portion of a weld slug welded to a first face surface of the disc element and a fulcrum member on the opposite, second face surface of the disc element. The fulcrum member has an edge engaging the second face surface spaced from the weld zone in a direction toward the second distal end to serve as a fulcrum to cause bending of the disc element upon reversing curvature at a location spaced from the weld zone. According to a first embodiment, an electrical contact is mounted to the disc element at the second distal end with the disc element serving as a current carrying member for use as a control element in a protector device responsive to over current conditions in a circuit serially connected to the disc element. According to another embodiment the disc element is used as a control element responsive to ambient temperature to transfer motion to a movable contact arm to open or close an electric circuit.
Other objects and features of the invention will become more readily understood from the following detailed description and appended claims, when read in conjunction with the accompanying drawings in which like reference numerals designate like parts throughout the figures thereof. Certain dimensions may have been modified for purposes of illustration.